| Nickel-free Ti22Nb6Zr shape memory alloy with certain superelasticity and goodbiocompatibility is one of bio-metal implant materials with board application prospects. Themechanical properties of porous Ti22Nb6Zr alloy are very close to that of natural bone,making it an ideal material for repairing and replacing human bones. The bio-activity ofTi22Nb6Zr alloy, however, is not good enough for bone implants. therefore bioactive surfacemodification is necessitated to increase its bio-activity to meet the requirements forreplacement materials of bone tissue.This thesis has investigated in details the processes of preparing TiO2/HA (hydroapatite)composite films on near dense (porosity6.7vol.%) and porous (porosity40vol.%)Ti22Nb6Zr alloys. Scanning electron microscope (SEM), energy dispersive X-ray spectrum(EDS) and X-ray diffraction (XRD) were used to characterize the surface morphology,chemical composition and structure of the films; Nano-indentation was used to test thefilm-substrate cohesion forces; Contact angles on the films were measured with contact angleinstrument to evaluate the hydrophilicity of the films; Corrosion resistance was evaluated bypolarization curves in0.9%NaCl aqueous solution.The results showed that an integrated and uniform titanium oxide film could be preparedon near dense Ti22Nb6Zr alloy by both high temperature oxidation process and alkalitreatment process. Film-substrate cohesion forces and corrosion resistance of the hightemperature oxidized film were far superior to that of the films prepared through alkalitreatment, while the result of hydrophilicity was just opposite. Optimized parameters of hightemperature oxidation process (600℃,1h) and alkali treatment process (4mol/L NaOH,70℃,5h) were selected through comprehensive analysis. Then the two optimized processesmentioned above were united to combined bio-active treatments, and it turned out that theprocess of high temperature oxidation followed by alkali treatment (O+A) was superior to theprocess of alkali treatment followed by oxidation (A+O), and the obtained titania film byO+A process increased film-substrate cohesion force effectively.Before HA deposition, pre-calcification process was added after combined bioactiveO+A treatment. The results showed that the thickness of the film prepared by bio-depositionfor21days with pre-calcification was about1.2μm,5times thicker than that withoutpre-calcification, suggesting that pre-calcification can accelerate bio-deposition rateeffectively. The film was consisted of two layers: the inner layer was titanium oxide (0.2μm)and the outer layer was HA layer (1μm). In addition, the film-substrate cohesion force with pre-calcification was50%higher than that without pre-calcification. Hence the optimalprocess flow of bio-active surface treatment was combined bioactive treatment (O+A)followed by pre-calcification and then HA bio-deposition in turn.When the above optimal process was applied to porous Ti22Nb6Zr alloy (porosity40vol.%), the bio-deposition rate was accelerated, due to the increase of specific surface areawhich helps to raise reaction rate. The thickness of the film on porous Ti22Nb6Zr alloys afterbio-deposition for7days was up to1.5μm, even thicker than that on dense alloy afterbio-deposition for longer time (21days,1.2μm) and the films both on surface and in poreswere integrated without cracking. The film on porous Ti22Nb6Zr alloy after bio-deposited for21days was8μm thick and cracked seriously. Therefore, it can be concluded that when theoptimal process for dense alloy was applied to porous alloy, the bio-deposition time should beappropriately reduced to prevent the over thickening and cracking of the film.Research also showed that the remarkable contribution to corrosion resistanceimprovement of Ti22Nb6Zr alloys came mainly from the combined O+A layer while theHA bio-deposition layer enhanced the corrosion resistance very slightly, whose primary rolewas to improve the bioactivity of the alloy. |
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